DPSK optical transmission has attracted much attention recently due to the unique advantages offered by DPSK modulation with balanced detection such as superior receiver sensitivity, high tolerance to some major nonlinear effects in high-speed transmissions, and high tolerance to coherent crosstalk.
The return-to-zero (RZ) pulse format, offering high receiver sensitivity and high immunity to inter-symbol interference, is well known to offer additional advantages over non-return-to-zero (NRZ) formats in most applications. RZ-DPSK has thus become the modulation format of choice for high-capacity, long-haul transmissions.
Existing Schemes for the generation of chirp-free RZ-DPSK signals typically require a Mach-Zehnder modulator (MZM) for pulse carving and another MZM for exact phase modulation. Such a format is discussed in A. H. Gnauck, “40-Gb/s RZ-differential phase shift keyed transmission,” Optical Fiber Communication Conference 2003 (OFC'03), Paper ThE1 which is incorporated herein by reference. To reduce the complexity and cost of an RZ transmitter, RZ on-off-keyed (RZ-OOK) signals have been generated using a single MZM and a differential amplifier. Such an implementation is discussed in Y. H. Kao et al, “10 Gb/s soliton generation for ULH transmission using a wideband GaAs pHemt amplifier,” OFC'03, Paper FF6, which is incorporated herein by reference. However, such an implementation cannot be used for the generation of RZ-DPSK signals because unlike an RZ-OOK signal which has two states, ‘0’ and ‘1’, in the optical field domain, an RZ-DPSK signal has three states, ‘1’, ‘0’, and ‘−1’. In addition, both the ‘1’ and ‘−1’ states need to be in an RZ format with a similar duty cycle.
Accordingly, a need still exists, for cost effective methods and apparatus for generating RZ-DPSK optical signals.